Broadly speaking, adhesives can be classified into three categories: water based; solvent based and hot melt adhesives. In the case of water based or solvent based adhesives, energy from the outside, in some fashion, is applied to the system to evaporate the water or the solvent in order that an adhesive layer may be formed. For the solvent based types, the requisite evaporation of the solvent also introduces the solvent into the environment which both requires energy and can be deleterious to the surrounding environment. Costly solvent recovery equipment is then required to control pollution. It is for these reasons that hot melt pressure sensitive adhesives (hereinafter called HMPSA) are highly desirable since the elimination of water and solvent leads to a lower energy requirement to form the adhesive layer with hot melt systems.
The key requirement for a HMPSA is that it should have good cohesive and tackifying properties at ambient conditions. The adhesive should also have good flow characteristics if it is to be used in the bulk state, otherwise it should possess sufficient consistency when dissolved in suitable cosolvents so that it can be coated or sprayed on the surfaces to be bonded or the backing used to form a tape or label. Usually the HMPSA is prepared from a mixture of tackifying resin, polymer and possibly a plasticizer (when required to soften the adhesive and enhance the aggressive tack).
Certain commercial block copolymers such as a Shell's Kraton.RTM. are widely used as the polymer in HMPSA. The adhesives prepared from blends incorporating these polymers have very good adhesive and strength properties at room temperature and can be processed by conventional melt coating and extrusion techniques because of their good flow characteristics. Because of this excellent combination of properties exhibited by the ABA type of block copolymer when B represents a polydiene or a polyolefin block and A represents a polystyrene block, e.g. a thermoplastic elastomer, the use of such polymers for various HMPSA applications is growing in the industry.
However, these referenced block copolymers when used in combination with known tackifying resins for HMPSA blends have a serious drawback when blended with a major proportion of resin. Under this condition in order to realize the tackifying properties of the petroleum resin component of the blend, it is necessary to incorporate substantial levels of a plasticizer, usually a naphthenic oil or a liquid resin whereby the aggressiveness of tack and other requisite properties of these tertiary blends can be controlled. The use of plasticizers results in a number of deleterious effects on the using of HMPSA compositions including long term degradation of adhesion.
The petroleum resins generally used as tackifiers for Kraton.RTM. type block copolymers include those produced according to the teachings of U.S. Pat. No. 3,577,398 wherein the polymeric resinous composition is obtained from the polymerization of a feedstock containing a mixture of piperylene, i.e. pentadiene-1,3 and 2-methyl-2-butene in the presence of anhydrous particulate aluminum chloride and a hydrocarbon diluent. The resinous products are reported as having softening points of about 80.degree. C. to about 110.degree. C. (col. 2, line 62) although upon steam stripping the softening points are raised to a range of about 90.degree. C. to about 110.degree. C. (col. 2, lines 69-70). Unfortunately, as indicated above, the use of a major proportion of such a resin as a tackifier for Kraton.RTM. type block copolymers requires from 5 to 30 weight percent of plasticizer in the tertiary blend to provide a useful pressure sensitive adhesive composition. Even more critical is the general unsuitability of such a tackifying resin for water-white saturated midblock thermoplastic elastomers since these resins are colored (not water-white) and have detrimental thermal and ultra-violet light stability.
It is an object of this invention to provide an improved petroleum resin for tackification of pressure sensitive adhesive blends containing a saturated mid block thermoplastic elastomer.